1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a system and method for preventing dropped calls.
2. Description of Related Art
FIG. 1 depicts a diagram of a portion of a typical wireless communications system 10, which provides wireless communications service to a number of wireless or mobile units 12a–c, that are situated within a geographic region. The geographic region serviced by a wireless communications system is divided into spatially distinct areas called “cells.” Each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell and other factors. A conventional cellular system comprises a number of cell sites or base stations 14a–d, geographically distributed to support transmission and receipt of communication signals to and from the wireless units. Each cell site handles voice communications over a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area. One cell site may sometimes provide coverage for several sectors.
A base station 14a–d comprises the radios and antennas that the base station uses to communicate with the wireless units in that cell and also comprises the transmission equipment that the base station uses to communicate with a Mobile Switching Center (MSC) 16. The Mobile Switching Center 16 is responsible for, among other things, establishing and maintaining calls between the wireless units 12a–c, between a wireless unit 12a–c and a wireline unit 18 via a public switched telephone network (PSTN) 20 or between a wireless unit 12a–c and a packet data network (PDN) 22, such as the Internet. Depending on the embodiment, a base station controller (BSC) can be a separate base station controller (BSC) 24 connected to several base stations 14a–c or a base station controller (not shown) located at each base station 14a–c which administers the radio resources for the base stations and relays information to the MSC 16.
When active, a mobile unit receives forward link signals from and transmits reverse link signals to (at least) one cell site or base station. Each active mobile unit is assigned a forward link on which it receives its forward link signals on at least one forward link channel and a reverse link on which it transmits reverse link signals on at least one reverse link channel. There are many different schemes for defining forward and reverse link channels for a cellular telephone system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different channels are distinguished by different spreading sequences that are used to encode different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular information stream from a received signal using the appropriate spreading sequence to decode the received signal.
A detailed sequence of activities are typically followed before the mobile unit can access the wireless communications system to establish or receive a call. As is known in the art, calls between a CDMA mobile unit and a base station typically employ several kinds of control channels. Initially, a pilot channel is employed to continually broadcast certain system synchronization and timing information to all mobile units in an area. After initial synchronization is achieved at a mobile unit, a sync channel is used to establish more specific time and frame synchronization at the mobile unit. The sync channel message also provides information about another class of channels, the paging channels. Paging channels are used to broadcast a variety of control information. Other overhead messages are sent between the mobile unit and the base station to facilitate communications over forward and reverse link traffic channels between the wireless unit and the base station over which voice and/or data information is sent.
In CDMA communications, the wireless unit searches for pilot signals of base stations on an active set, a candidate set and a neighbor set. The active set is the set of base stations through which active communications is established. The neighbor set is the set of base stations surrounding an active base station having some probability of having a signal strength of sufficient level to establish communications, and the candidate set is a set of base stations having a pilot signal strength at a sufficient level to establish communication with the wireless unit with a probability close to the active set. The wireless unit measures the signal strengths of the pilot signals and provides the pilot signal measurements in a pilot strength measurement message (PSMM) to the wireless communications system through a serving base station. The wireless unit determines which base stations are in the candidate set based on the pilot signal strength measurements. The wireless units sends the pilot signal measurements for the candidate and active base stations in the PSMM. Receipt of the PSMM is acknowledged by the wireless communication system to the wireless unit. When a pilot signal of a base station in the neighbor set exceeds a predetermined threshold level, the base station is added to the candidate set. When the wireless unit detects a pilot of sufficient strength which is associated with a base station in the candidate set, the wireless communications system determines whether to update the active set and assign a traffic channel from the base station to the wireless unit. The wireless unit is said to be in soft handoff if it is assigned traffic channels from more than one base station.
All base stations connected to a given wireless unit define the active set of that mobile, and an active set update function controls, i.e., evaluates and updates, this active set based on the pilot strength measurements. At the wireless unit, the strongest pilots are detected and measured by the measurement process. The signal strength values are then collected into the pilot measurement report, which is sent to the wireless communications system. When the active set update function is invoked, the base stations within the active set from which the strongest and weakest pilot are received are identified. If the difference is greater than the system desired value (called active set window), the weakest base station will be removed from the active set. A base station is added to the active set window if its received pilot signal strength is within the window above an active set threshold value, provided the active set size is not exceeded. If the active set size is full, the weakest base station in the active set will be replaced by the new base station if the corresponding pilot signal strength is higher than the weakest base station.
Currently, when a call is in progress, the wireless unit continuously monitors the forward traffic channel. If the wireless unit receives a signal of inadequate quality over a period of 12*20 ms (or 12 bad frames) on the forward channel from the serving base station(s) in the active set, the wireless unit turns off its transmitter on the traffic channel and starts counting down a five second fade timer. The wireless unit retransmits again and resets the fade timer to 5 seconds if it receives sufficient signal quality for a period of 2*20 ms (or 2 consecutive good frames) from the same base station(s) before the fade timer expires. If the fade timer expires, the wireless unit declares a loss of forward traffic channel and terminates the call. It is possible however that even as the fade timer is running down, the wireless unit changes its location. This may in turn make it possible for the call to be sustained via other base stations in the vicinity of the wireless unit that were not previously serving this call. However, due to the broken communication link between the wireless unit and the serving base station(s), it is not currently possible in CDMA communications systems, for either the network or the mobile to take action to recover the call via this new set of base stations. This is because any change in the serving base station(s) in the active set relies of the availability of a communication link between the wireless unit and the current set of serving base station(s).
For example, the wireless unit can only communicate on the traffic channel with the serving base station(s) in the active set. In order to propose changes to the active set, the wireless unit sends the pilot strength measurements (for example, using the Pilot Strength Measurement Message (PSMM)) to the serving base station(s). The base station may then send a handoff direction to direct the wireless unit to change the active set. If the communication link between the mobile station and the serving base stations breaks any time during this process, the serving base station(s) in the active set will not be successfully changed, and the call may be dropped if the wireless unit cannot reestablish the call with the current set of serving base station(s) in the active set.